1. Field of the Invention
The invention relates generally to microwave devices and, more particularly, to vertical interconnections between coplanar waveguides.
2. Description of the Related Art
A conventional flip-chip microwave device includes a monolithic microwave integrated circuit (MMIC) flip-mounted on an assembly substrate. Either the MMIC and/or the assembly substrate typically has metal bumps to couple the MMIC to output circuits disposed on the assembly substrate. This flip-chip MMIC technology has provided numerous advances in microwave device fabrication, not the least of which involves utilizing the surface tension of the molten bump metal (i e., solder) to align accurately the MMIC and the output circuit of the assembly substrate.
Such microwave devices commonly include coplanar waveguides for low loss signal transmission in both the MMIC and the output circuit on the assembly substrate. At a vertical transition between the MMIC and the output circuit, bumps are provided for connecting the respective center conductors and ground planes of the two coplanar waveguides. Such vertical interconnections have utilized round or square bumps in a ground-signal-ground configuration dictated by the location of the center conductors and the ground planes. As a result, the signal-carrying bump has typically been disposed between the two ground bumps.
The advantages provided by the use of coplanar waveguides in both the MMIC and the output circuit are hampered by these vertical interconnections. Interest in operating such flip-chip devices at ever higher microwave frequencies has introduced radiation losses as the spacing between the bumps becomes a significant fraction of the wavelength of the transmitted signals. For example, once the distance between the inner edges of the ground bumps approaches one-half of the wavelength, such radiation losses arise from the vertical transition circuit behaving similar to a half-wavelength dipole radiating element. Radiation losses may arise from propagation through the MMIC substrate as well as through free space, in which case the relevant wavelength is appropriately adjusted by the dielectric constant of the substrate. In general, these radiation losses have limited the operating frequency of current microwave devices.
Other losses arise from vertical transition impedance mismatch. These vertical interconnections are typically inductive in nature, as it is difficult to dispose the ground bumps close enough to the signal-carrying bump to achieve significant capacitance therebetween. A series resistor-inductor lumped equivalent circuit results and, depending on the impedance of the coplanar waveguides, a potentially highly undesirable circuit discontinuity may exist at each vertical transition. The impedance mismatch between the coplanar waveguides and the vertical transition circuit becomes more pronounced as the operating frequency increases, inasmuch as the impedance of a series inductor increases linearly with frequency. Still further, impedance mismatch increases at an even greater rate as the wavelength of the operating frequency approaches the spacing between the bumps.
Micromachined membrane devices, which typically include one or more thin silicon substrates having circuitry disposed on both sides, have encountered similar complications in connection with the vertical interconnections passing through each substrate. These vertical interconnections (in this case, vias) are indispensable for connecting the circuits disposed on opposite sides of the same substrate. To the extent that such micromachine devices have included coplanar waveguides for transmitting microwave signals, the devices have utilized ground-signal-ground configurations for each via similar to the bump interconnections used in connection with flip-chip modules. Consequently, the same impedance mismatch and radiation loss problems thwart attempts to operate micromachined membrane devices at higher frequencies.